544 THE PALEONTOLOGY OF MINNESOTA. 



[Cyrtodonta pesrslrallis . 



Formation and locality. The original type is from the middle third of the Trenton shales at Minne- 

 apolis. A small cast of the interior, belonging to the survey collection, was found in the building lime- 

 stone at the same place. Casts occur also in the lower Trenton limestone at Beloit, Wisconsin and Dun- 

 leith, Illinois. 



Mus. Reg. No. 5100. 



CYRTODONTA PERSIMILIS, n. sp. 



PLATE XXXIX, FIGS. 41 and 44. 



This form, which is known only from casts of the interior, was confused with 

 C. glabella until a critical comparison proved it to be not only distinct but to belong 

 to another group of species. The outline is very much alike in the two species, but 

 here even some constant differences are to be observed, especially in the shape of 

 the margin at the posterior extremity of the hinge, where the present species is 

 more angular. But the main difference lies in the fullness of the umbones, there 

 being no appreciable sign of the sulcus and ridge which cross this portion of casts 

 of that species. This difference is very obvious after it has once been pointed out. 

 The beaks are also more strongly incurved and the hinge bent downward anteriorly 

 in a greater degree, while the plate is probably also of less width. Finally, the 

 posterior muscular scar is closer to the hinge and the longer diameter of the 

 impressions more oblique. 



The systematic position of the species is near C. rotulata, C. cingulata, and C. 

 tenella. The first is more rotund in outline, less oblique and has fuller umbones, 

 the others are higher and have the beaks situated farther behind the anterior 

 extremity. 



At Minneapolis C. persimilis is associated with a small Vanuxemia that is not 

 easily distinguished unless the casts are clean and in good condition. The latter 

 (V. decipiens,)differB somewhat in its outline being proportionally narrower anteriorly, 

 but the principal difference lies in the character of the anterior adductor scar, which 

 is much more distinct from the umbonal cavity. In short, the species is not a 

 Crytodonta but a Vanuxemia as now defined. 



Formation and Locality. Trenton limestones, Minneapolis, Minnesota, "Lower Blue Hods" of the 

 Trenton formation at lir.loit, Wisconsin. 



CYRTODONTA OVIFORMIS, Ulrich. 



PI, ATI: \\\i.\. Kit;. i.; ; IM.ATFO XL. FIG. i. 

 Cypricarditex oviformis I'LKK n. 1802. Amer. Geol., vol. x, j>. !)!i. 



Shell rather above the medium in size, moderately convex, but little oblique, the 

 outline almost regularly oval, with the posterior end a little the widest and a slight 

 straightening along the cardinal margin. Beaks small, situated between one-fourth 

 and one-fifth of the length behind the anterior extremity; erect, compressed and not 



